Looking to buy K75, K100, K1100 rear master cylinder from a seller in Europe, need to be posted to Croatia.

Doesn't matter if it's 12 or 13 mm

Have you tried Motorworks in the UK?

I searched a while ago and couldn't find any. With a bit of modification you can use a Chinese one. See link   http://www.motobrick.com/index.php/topic,9364.msg76252.html#msg76252 Do not use the supplied reservoir hose it will eventually fail dumping brake fluid onto your rear tyre. I've been using one since March 2017 with no issues if anything it has better feel.
Regards Martin.

Thanks for both advices - Motorworks does not have any at the moment, but I will consider Chinese if not possible to locate a 2nd hand one

How about Motobins or even James Sherlock

Pete Riley wrote:How about Motobins or even James Sherlock

Motobins does not store used parts, just new. Sherlock may have them but I am unsure would they post to Croatia, where I need it delivered

After Brexit (UK leaving the EU) it has become much more troublesome for EU residents to buy from the UK.

I don't know the specifics of the two companies above.

Going the Chinese route is my kind of thing

A slight thread drift about buying from Chyna.  

My father recently purchased some so called knock off Ryobi 7 Amp Hour batteries for some power tools from an address in Sydney through ebay.  Long story short, they did not last as advertised and were lighter than the standard battery.  A bit of a ding dong email battle with the seller and he eventually received a full refund and kept the two batteries so the seller did not receive a bad online rating.  

So he pulled one apart and sure enough half the batteries were missing.  So a typical student from Chyna, sponsored to study in Australia, as long as you run a business for your sponsor whilst here.  

Not tarring the brush of all international students but just aware.